1. Field of the Invention
This invention relates to a method for purifying an exhaust gas in a wet mode, and more particularly, to a method for preventing scale formation in a wet type exhaust gas treating apparatus.
2. Discussion of the Background
A wet type exhaust gas treating apparatus is an apparatus for contacting an exhaust gas with a washing liquid containing water to remove fine particles, a water-soluble substance, and a water-decomposable substance in the exhaust gas. The wet type exhaust gas treating apparatus uses, for example, a packed column, a spray tower, a gas passage tank, and an agitating gas passage tank.
As the washing liquid, tap water, or an aqueous solution containing some chemical in tap water is generally used. Such a wet type exhaust gas treating apparatus is often continuously run, and often circulates the washing liquid to decrease the amount of a waste liquor discharged to the environment.
With the wet type exhaust gas treating apparatus involving the circulation of a washing liquid, it is known to supply a constant amount of water to the washing liquid and simultaneously discharge the same amount of water in order to permit a continuous run while preventing the reaction product between the washing liquid and a harmful component in the exhaust gas from building up in the system.
However, there may be a buildup of insolubles, i.e., scale, in the washing liquid. If the insolubles form, they may adhere to a gas-liquid contact area, a circulating pump for the washing liquid, a piping, etc., causing a trouble such as blockage.
Particularly, it will present a problem if feed water to be supplied to the washing liquid contains ions which react with a harmful component in the exhaust gas to form an insoluble compound. For example, if the washing liquid is kept neutral or alkaline and the feed water contains iron ions, insoluble iron hydroxide may be formed. If the feed water supplied to the washing liquid contains calcium ions and the exhaust gas contains a fluorine gas (F2) or carbon dioxide (CO2), on the other hand, insoluble calcium fluoride and/or calcium carbonate may be formed in the washing liquid. Borate ions or silicate ions may also become the cause of scale.
An exhaust gas containing a fluorine gas (F2) and carbon dioxide (CO2) may be discharged from an apparatus for production of a semiconductor device. For example, when dry etching of a silicon wafer or the like is performed with the use of a semiconductor device manufacturing apparatus such as an etching apparatus, fluorinated hydrocarbons, such as CF4, CHF3 and C2F6, are used. An exhaust gas from the etching apparatus contains a fluorine gas (F2) and carbon dioxide (CO2), decomposition products of the fluorinated hydrocarbons.
During cleaning of the apparatus for producing a semiconductor device, an exhaust gas containing a fluorine gas (F2) may be discharged. When a thin film is to be formed on a semiconductor substrate, a chemical vapor deposition apparatus is used as a semiconductor device manufacturing apparatus. In cleaning a thin film, adhered to the inner surface of a chamber or a piping of the chemical vapor deposition apparatus, with a ClF3 gas, an exhaust gas containing a fluorine-containing gas, such as F2, SiF4, BF3 or PF3, is discharged from the chemical vapor deposition apparatus.
Furthermore, an exhaust gas from a semiconductor manufacturing apparatus may contain a boron compound such as BCl3 or BF3, or a silicon compound such as tetraethoxysilane (hereinafter referred to as TEOS) or SiH2Cl2. The boron compound and silicon compound react with water in the washing liquid, and decompose into borate ions and silicate ions. These borate ions and silicate ions cause scale formation.
The system involving the formation of such insoluble products has required periodical washing and replacement of some parts for the purpose of preventing troubles. Maintenance of a treating apparatus is tiresome, and the parts to be replaced are costly. It is occasionally performed to withdraw part of the washing liquid, thereby keeping the concentration of the insoluble compound in the washing liquid at a level not higher than its solubility. However, this procedure increases the amount of drainage, and is not preferred.
Japanese Unexamined Patent Publication No. 122470/1975 describes the prevention of scale deposition on the internal surface of a boiler by adding a chelant-surfactant to an aqueous system. Japanese Unexamined Patent Publication No. 92888/1996 describes the prevention of scale by adding citric acid or its salt in a deinking process for waste paper.
Neither publication, however, describes a method for treating an exhaust gas. Nor do these publications describe a method for treating an exhaust gas discharged from an apparatus for producing a semiconductor device.
A first aspect of the present invention aims to resolve the foregoing problems, and prevent the formation of an insoluble compound in a washing liquid. A second aspect and a third aspect of the invention are designed to produce a semiconductor device while treating an exhaust gas without forming an insoluble compound in a washing liquid.
In all aspects of the invention, a chelating agent in a washing liquid is coordinated with ions to form a water-soluble chelate compound, thereby preventing the formation of an insoluble product.
According to the first aspect of the invention, there is provided a method for purifying an exhaust gas in a wet mode, comprising the step of contacting the exhaust gas with a washing liquid containing water, the step of circulating the washing liquid to reuse it, the step of supplying a constant amount of a feed liquid per unit time to the washing liquid, and the step of discharging the constant amount of the washing liquid per the unit time from the washing liquid, the washing liquid containing at least one chelating agent which reacts with ions becoming a cause of scale formation to form a water-soluble chelate compound.
According to the second aspect of the invention, there is provided a method for producing a semiconductor device, comprising the step of etching a precursor of the semiconductor device with an etching gas or its plasma in a chamber, the step of discharging the etching gas or its plasma from the chamber, the step of contacting an exhaust gas discharged from the chamber with a washing liquid containing water, the step of circulating the washing liquid to reuse it, the step of supplying a constant amount of a feed liquid per unit time to the washing liquid, and the step of discharging the constant amount of the washing liquid per the unit time from the washing liquid, the washing liquid containing at least one chelating agent which reacts with ions becoming a cause of scale formation to form a water-soluble chelate compound.
According to the third aspect of the invention, there is provided a method for producing a semiconductor device, comprising the step of chemical vapor depositing a thin film-forming gas on a precursor of the semiconductor device in a chamber of a chemical vapor deposition apparatus to form a thin film, the step of withdrawing the resulting semiconductor device from the chamber, the step of cleaning the chamber of the chemical vapor deposition apparatus with a cleaning gas, the step of contacting an exhaust gas in the cleaning step, which has been discharged from the chamber of the chemical vapor deposition apparatus, with a washing liquid containing water, the step of circulating the washing liquid to reuse it, the step of supplying a constant amount of a feed liquid per unit time to the washing liquid, and the step of discharging the constant amount of the washing liquid per the unit time from the washing liquid, the washing liquid containing at least one chelating agent which reacts with ions becoming a cause of scale formation to form a water-soluble chelate compound.
According to a fourth aspect of the invention, there is provided a method for preventing scale formation in a wet type exhaust gas treating apparatus which treats an exhaust gas containing a fluorine gas and carbon dioxide by contacting the exhaust gas with a washing liquid, characterized in that at least one chelating agent which reacts with ions becoming a cause of scale formation to form a water-soluble chelate compound is added to the washing liquid.
Preferably, the invention further has the step of adding an aqueous solution containing the chelating agent to the washing liquid or the feed liquid. Alternatively, it is preferred for the invention to further have the step of adding a chemical solution containing water and the chelating agent to the washing liquid or the feed liquid. Alternatively, it is preferred that the feed liquid be obtained by the step of passing water through a column packed with a solid chelating agent having suitable solubility.
It is also preferred that the exhaust gas has been generated from a semiconductor device manufacturing apparatus.
Preferably, the exhaust gas contains an acidic gas, and the claimed method further has the step of adding a basic chemical solution to the washing liquid or the feed liquid. Alternatively, it is preferred that the exhaust gas contains a basic gas, and the claimed method further has the step of adding an acidic chemical solution to the washing liquid or the feed liquid.
Further preferably, a packed column, a spray tower, a gas passage tank, or an agitating gas passage tank is used in the contacting step.
Preferably, the ions are metal ions. Alternatively, the ions are preferably iron ions, calcium ions, borate ions, or silicate ions.
Furthermore, the chelating agent is preferably an aminocarboxylic acid or its salt, an oxycarboxylic acid or its salt, a silicate, or a polyphosphate.
Moreover, it is preferred for the claimed method to have the step of adding a chemical solution containing a pH adjustor to the washing liquid or the feed liquid. It is further preferred for the method to have the step of measuring the pH of the washing liquid, and the step of controlling the amount of the chemical solution containing the pH adjustor based on the measured value of pH.